Alumina-magnesia product for gasifier or for metallurgical furnace

Abstract

The invention relates to a melted and cast refractory product having a chemical composition such that, in mass percentages on the basis of the oxides: AI.sub.2O.sub.3: complement up to 100%; MgO: 26% to 50%; ZrO.sub.2: 0.5% to 10.0%; B.sub.2O.sub.3: <1.5%; SiO.sub.2: 0.5%; Na.sub.2O+K.sub.2O: 0.3%; CaO: 1.0%; Fe.sub.2O.sub.3+TiO.sub.2: <0.55%; other oxide species: <1.0%. In said product, the elementary mass ratio R of the zirconium content to the total boron, fluorine and silicon content is between 2 and 80.

Claims

1. A fused-cast refractory product having a chemical composition such that, in percentages by weight on the basis of the oxides: Al.sub.2O.sub.3: balance to 100%; MgO: 26% to 50%; ZrO.sub.2: 0.5% to 10.0%; B.sub.2O.sub.3: 1.5%; SiO.sub.2: 0.5%; Na.sub.2O+K.sub.2O: 0.3%; CaO: 1.0%; Fe.sub.2O.sub.3+TiO.sub.2: <0.55%; other oxide species: <1.0%; provided that an elementary weight ratio R of the content of zirconium Zr to the total content of boron B. fluorine F and silicon Si is between 2 and 80.

2. The product as claimed in claim 1, wherein the weight content of ZrO.sub.2 is greater than or equal to 1.0%.

3. The product as claimed in claim 2, wherein the weight content of ZrO.sub.2 is greater than or equal to 2.0%.

4. The product as claimed in claim 1, wherein the weight content of ZrO.sub.2 is less than or equal to 7.0%.

5. The product as claimed in claim 4, wherein the weight content of ZrO.sub.2 is less than or equal to 5.0%.

6. The product as claimed in claim 1, wherein more than 70% by volume of the zirconia is present in monoclinic form.

7. The product as claimed in claim 1, wherein the weight content of B.sub.2O.sub.3 is greater than or equal to 0.05%.

8. The product as claimed in claim 7, wherein the weight content of B.sub.2O.sub.3 is greater than or equal to 0.1%.

9. The product as claimed in claim 8, wherein the weight content of B.sub.2O.sub.3 is greater than or equal to 0.2%.

10. The product as claimed in claim 1, wherein the weight content of B.sub.2O.sub.3 is less than or equal to 0.6%.

11. The product as claimed in claim 10, wherein said elementary weight ratio R is between 5 and 50.

12. The product as claimed in claim 11, wherein said elementary weight ratio R is between 7 and 30.

13. The product as claimed in claim 1, wherein the weight content of CuO is less than or equal to 0.4%.

14. The product as claimed in claim 1, wherein the weight content of alumina Al.sub.2O.sub.3 is less than or equal to 70% and greater than or equal to 55%, and/or the weight content of MgO is less than or equal to 40% and greater than or equal to 29%.

15. The product as claimed in claim 14, wherein the weight content of alumina Al.sub.2O.sub.3 is less than or equal to 68% and greater than or equal to 60%, and/or the weight content of MgO is less than or equal to 35% and greater than or equal to 32%.

16. The product as claimed in claim 1, wherein the weight ratio of Al.sub.2O.sub.3 to MgO is less than 2.6 and greater than 1.2.

17. The product as claimed in claim 1, wherein the weight content of CaO is less than or equal to 0.6%; and/or the weight content of Na.sub.2O+K.sub.2O is less than or equal to 0.25%; and/or the weight content of silica SiO.sub.2 is less than or equal to 0.15%; and/or the weight content of iron and/or titanium oxides, Fe.sub.2O.sub.3+TiO.sub.2, is less than 0.4%; and/or the weight content of chromium oxide is less than 0.1%, and/or the weight content of CuO is less than or equal to 0.1%.

18. A device selected from: a gasifier, a metallurgical furnace, an anode baking furnace, a cupola furnace, a municipal waste incinerator, a glass furnace, a regeneration chamber of a glass furnace, an electrolytic cell for the electrolysis, in a molten salt medium, of nonferrous metals, a device for protecting or regulating streams of cast iron or steel, an agitation device, either of mechanical or gas-injection type, for agitation in a molten metal, a seating block serving as a housing and support for a gas-injection device or for an injection device for regulating a metal stream, an impact tile for ladles or tundishes, a foundry accessory for cast iron, steel and special steels, a support for firing ceramic products, a regenerator, said device comprising a product as claimed in claim 1.

19. An assembly comprising a device as claimed in claim 18, and a material selected from a slag, coal, a corrosive liquor, a molten metal, a molten glass, alkaline vapors, liquid condensates resulting from the melting of glass, and an abrasive material, said product being in contact with said material.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) In a fused-cast product according to the invention, the combination of a high alumina content with an MgO content greater than 26% makes it possible to obtain a product of crystalline structure comprising predominantly Al.sub.2O.sub.3MgO spinel capable of meeting the requirements of high resistance to slag, to corrosive liquors and to certain molten glass, in particular soda-lime glass or else boron-loaded glass, especially borosilicate glass.

(2) The inventors have found that a presence of zirconia ZrO.sub.2 according to the invention advantageously enables products to be obtained that have a low porosity, that withstand thermal cycling well and that do not exude at high temperature, provided that the content of ZrO.sub.2 is between 0.5% and 10% by weight.

(3) The inventors have also discovered that the combined presence of zirconia ZrO.sub.2 and boron, for example in B.sub.2O.sub.3 form, advantageously makes it possible to obtain an optimum for the desired properties. The inventors have observed that the zirconia is then mainly in monoclinic form. This form of the zirconia is surprising for a product synthesized by melting at high temperature. Indeed, such melting normally results in the presence of zirconia in cubic form.

(4) The ratio R=Zr/(B+F+Si) is between 2 and 80. It is preferably greater than or equal to 4, preferably greater than or equal to 5, preferably greater than or equal to 7, or even greater than or equal to 9, and/or preferably less than or equal to 50, preferably less than or equal to 40, preferably less than or equal to 30, or even less than or equal to 20.

(5) An elementary weight ratio R which is too low results in a product that is insufficiently densified and potentially porous. If the elementary weight ratio R is too high, the product with equivalent density is more sensitive to thermal shock and to corrosion with respect to certain aggressive agents such as alkali metals.

(6) The content of silica SiO.sub.2 must not exceed 0.5% as it may impair the corrosion resistance. In particular, with too high a content, the silica could combine with the impurities to increase the glassy phase, harmful to corrosion resistance, or react with alumina to form mullite. Yet, the presence of mullite is prejudicial to the resistance to corrosion by papermaking black liquors.

(7) In a product according to the invention, the oxides Na.sub.2O and K.sub.2O are considered to have similar effects. The oxides Na.sub.2O and K.sub.2O have an unfavorable effect on the resistance to the formation of beta-alumina and tend to increase the content of the glassy phase. The weight content of Na.sub.2O+K.sub.2O must therefore be less than or equal to 0.3%.

(8) The weight content of CaO must also be less than or equal to 1.0%, preferably less than or equal to 0.4%.

(9) According to the invention, the weight content of Fe.sub.2O.sub.3+TiO.sub.2 is less than 0.55% and, preferably, the weight content of Cr.sub.2O.sub.3 is less than 0.2%, preferably less than 0.10%. This is because these oxides are deleterious and their content must preferably be limited to traces, preferably introduced as impurities with the raw materials.

(10) The other oxide species are species other than Al.sub.2O.sub.3, MgO, ZrO.sub.2, B.sub.2O.sub.3, SiO.sub.2, Na.sub.2O, K.sub.2O, CaO, Fe.sub.2O.sub.3 and TiO.sub.2. In one embodiment, the other oxide species are limited to species whose presence is not particularly desired and which are generally present as impurities in the raw materials.

(11) Conventionally, in a fused-cast product such as a product according to the invention, the oxides represent more than 98.5% or more than 99% or even substantially 100% of the weight of the product. The same applies in a product according to the invention.

(12) A product according to the invention may have a complex shape. In particular, it may take the form of a brick or block having a nonplanar surface, for example a concave and/or convex surface, especially a cylindrical, conical or angled surface. A complex shape makes it possible in particular to fit the arrangement of the members of a gasifier and in particular for ducts to pass therethrough.

(13) A product according to the invention may be in the form of a block having dimensions of greater than 100 mm100 mm100 mm.

(14) In one embodiment, the product is in the form of a block having a weight of greater than 5 kg. It may have a shrinkage cavity, i.e. a macroscopic porous volume located in one portion of the block if this portion has not been removed by machining of the solidified bloc. This shrinkage cavity is linked to the shrinkage of the product during the solidification. The portion of the block outside of the shrinkage cavity is densified. This densest zone of the block, referred to as the sound zone, preferably represents at least 10%, preferably at least 20% by volume of the block. This zone preferably has an open porosity of less than 7%, preferably less than 6%, preferably less than 5%, or even less than 3%.

(15) The bulk density of the sound zone is preferably greater than 3.20 g/cm.sup.3, more preferably greater than 3.25 g/cm.sup.3, more preferably greater than 3.30 g/cm.sup.3, or even greater than 3.40 g/cm.sup.3, or even greater than 3.45 g/cm.sup.3, or even greater than 3.50 g/cm.sup.3.

(16) In the sound zone, the various oxides are distributed substantially homogeneously. In particular, there is no systematic change (increase or decrease) in an oxide close to the outer surface of the product.

(17) A product according to the invention may be conventionally manufactured via the steps a) to c) described below: a) mixing of raw materials so as to form a feedstock; b) melting of said feedstock so as to obtain a molten material; and c) solidification of said molten material, by cooling, so as to obtain a refractory product according to the invention.

(18) In step a), the raw materials are chosen so as to guarantee the oxide contents in the end product.

(19) Preferably, the oxides for which a minimum content is necessary, especially Al.sub.2O.sub.3, MgO and ZrO.sub.2 or precursors of these oxides, for example AlF.sub.3, are added systematically and methodically. AlF.sub.3 advantageously facilitates the onset of melting and limits the risks of cracking.

(20) Preferably, the contents of these oxides in the sources of the other oxides are taken into account.

(21) Where appropriate, boron may be introduced in the form of B.sub.2O.sub.3 or by any precursor, including B.sub.4C or ZrB.sub.2, or even CaB.sub.6, preferably in a form that limits excessively rapid fly-off during melting.

(22) Fluorine is preferably provided in the form of a fluoride of an element whose presence is necessary in a product according to the invention, for example in the form MgF.sub.2 or ZrF.sub.4. It may also be provided, for example, in the form of KBF.sub.4, KF or NaF or of ores comprising at least one of these fluorides.

(23) In step b), the melting is preferably carried out by combining the action of quite a long electric arc, causing no reduction, with stirring, which promotes reoxidation of the products. Short or moderately short arc adjustment can also be used for producing the product.

(24) To minimize the formation of nodules with a metallic aspect and to avoid formation of cracks or crazes in the end product, it is preferable to carry out the melting operation under oxidizing conditions.

(25) Preferably, the long-arc melting process described in French patent No. 1 208 577 and its additions No. 75893 and No. 82310 is used.

(26) This process consists in using an electric arc furnace in which the arc is struck between the charge and at least one electrode away from this charge, and in adjusting the length of the arc so that its reducing action is minimized, while still maintaining an oxidizing atmosphere above the molten bath and stirring said bath.

(27) The melting operation may in particular be carried out at a temperature above 2000 C., preferably between 2050 C. and 2200 C.

(28) In step c), the cooling can be varied according to the molding technology necessary for producing the parts.

(29) Preferably, in step c), the casting temperature is greater than or equal to 2000 C.

(30) In step c), two routes are possible: casting into a mold wedged in a formwork using a granular insulator. The cooling is then natural, the process being called a transferless process; and casting into a mold which is opened very rapidly to extract the part and to insert it into a box containing an insulator. The part is then immediately covered with insulator, the process being called a transfer process.

(31) In the latter technology, the cooling is controlled, preferably so as to be carried out at a rate of less than 20 C. per hour, preferably at a rate of about 10 C. per hour.

(32) A product of the invention thus manufactured mainly consists of alumina-magnesia spinel and periclase crystals. Preferably, a product according to the invention comprises less than 10%, less than 5%, less than 2%, less than 1%, in percentages by weight, or even substantially no alumina crystals.

EXAMPLES

(33) The following nonlimiting examples are given for the purpose of illustrating the invention.

(34) In these examples, the following raw materials were employed: calcined alumina mainly containing, as weight average, 99.5% Al.sub.2O.sub.3, 0.27% Na.sub.2O and 100 ppm SiO.sub.2; AlF.sub.3 containing at most 0.15% SiO.sub.2 and at most 0.25% Na.sub.2O; calcined high-purity synthetic magnesia, containing 98.5% MgO, at most 0.9% CaO, 0.2% SiO.sub.2 and at most 0.6% Fe.sub.2O.sub.3; boron carbide, containing 76.25% boron, 20.1% carbon and 0.18% Fe.sub.2O.sub.3; zirconia CC10 supplied by SEPR having a median diameter of around 3 microns; and copper oxide Cu.sub.2O typically containing 98.7% Cu.sub.2O and 0.7% metallic copper.

(35) The raw materials were melted using the conventional arc-furnace melting process, as described above, and then the molten material was cast to obtain blocks.

(36) The chemical analysis of the products obtained is given in table 1, namely an overall average chemical analysis, given in percentages by weight. The elemental chemical analyses are carried out by x-ray fluorescence. The boron is assayed by inductively coupled plasma (ICP) spectrometry, and the fluorine is assayed by ion chromatography after extraction by pyrohydrolysis.

(37) In table 1 below, * indicates that the example is outside the invention and an empty box corresponds to a content less than or equal to 0.04% by weight. The balance to 100% consists of the oxide species other than those mentioned in the table.

(38) The total porosity is given by the following equation:
Total porosity=100(absolute densitybulk density)/absolute density

(39) The bulk density and the open porosity are measured according to the ISO 5017 standard on a bar withdrawn from the core of the block, in the sound zone.

(40) The absolute density is measured on ground powder by means of a helium pycnometer.

(41) The resistance to thermal cycling is measured in the following manner: samples having a size of 505050 mm.sup.3 are prepared by cutting from the block in the sound zone. Five samples for each example are placed in a furnace at 1100 C. in air for 30 minutes then submerged in water at 20 C. for 5 minutes. The test is repeated until failure of the samples. Table 1 provides the mean number of cycles that the 5 samples withstood.

(42) X-ray diffraction analysis revealed the presence predominantly of spinel and periclase phases. The form of the zirconia was identified by x-ray diffraction. C stands for cubic, n.m. stands for not measured and NA stands for not applicable.

(43) TABLE-US-00001 TABLE 1 1* 2* 3* 4* 5 6 7 8 9 10 11 12 13* Chemical composition on the basis of the oxides Al.sub.2O.sub.3 71.4 63.24 62.82 61.88 62.4 60.6 62.4 66.0 61.6 65.1 58.6 63.6 56.4 MgO 27.6 35.2 36 35.91 34.6 38.0 34.6 30.7 34.1 29.1 34.2 26.0 31.4 ZrO.sub.2 <0.1 <0.1 <0.1 <0.1 2.6 0.5 2.2 2.5 2.9 4.8 5.6 9.1 11.3 B.sub.2O.sub.3 0.3 0.6 0.3 0.5 <0.05 0.4 0.3 0.3 0.7 0.4 0.9 0.8 0.4 SiO.sub.2 0.08 0.05 0.08 0.05 <0.05 0.05 <0.05 0.12 <0.05 0.13 <0.05 <0.05 0.07 Na.sub.2O + K.sub.2O 0.11 0.09 0.13 0.3 0.09 0.11 0.09 0.08 0.09 <0.05 0.10 0.11 0.08 CaO 0.14 0.35 0.05 0.5 0.18 0.23 0.28 0.14 0.28 0.15 0.27 0.13 0.2 Fe.sub.2O.sub.3 + TiO.sub.2 0.11 0.18 0.13 0.21 0.13 0.11 0.10 0.16 0.16 0.15 0.15 0.20 0.12 CuO 0.13 0.49 0.19 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 <0.05 Other oxides Balance to 100% Form of the zirconia NA NA NA NA C n.m. Monoclinic zirconia C Zr/(B + F + Si) <0.3 <0.2 <0.4 <0.4 >40 2 13 11 9 17 13 24 47 Bulk density 3.44 3.43 3.35 3.34 3.44 3.51 3.50 3.55 3.50 3.51 3.53 3.53 (g/cm.sup.3) Total porosity (%) 10.9 3.8 4.5 6.5 Open porosity (%) 9.5 2.8 3.0 3.5 7.2 1.7 0.7 1.5 0.6 3.4 1.8 1.2 2.2 Resistance to 0.7 0.6 0.8 4 2.5 2.6 3.0 2.9 2.8 2.5 2.0 1.8 thermal cycling

(44) Exudation was observed on the blocks from examples 2 to 4 after heat treatment at 1100 C. for 30 minutes. The outer surface of the block observed has greasing formed by blisters of greenish color composed of alumina, magnesia, silica and copper. This phenomenon was not observed on the blocks of the examples according to the invention.

(45) The results show that the tested products of the invention have an improved resistance to thermal cycling with respect to those of the comparative examples. Furthermore, the products according to the invention have an excellent compromise between the open porosity and the resistance to thermal cycling for the compositions comprising zirconia and boron.

(46) The results also show that the prior art compositions comprising CuO may be advantageously replaced by a composition according to the invention in order to obtain a product that is simultaneously not very porous, withstands thermal cycling better and does not have exudation problems.

(47) Of course, the present invention is not limited to the embodiments described, these being provided as illustrative and nonlimiting examples.